The present invention relates to the stripping of tin or tin-lead alloy layers from substrate surfaces, and more particularly to a method for regenerating tin or tin-lead alloy stripping compositions.
In the course of manufacturing printed circuit boards, it is commonplace to deposit (e.g., by electroplating, immersion or other like processes) a layer of tin or tin-lead alloy (solder) on all or selected conductive copper surfaces of the board defining traces, through-holes, surrounding pad areas and the like, to serve, for example, as an etch resist in the subsequent etching away of other copper surfaces. By the same token, it is necessary to eventually strip the tin or tin-lead alloy from all or selected copper surfaces coated therewith, as is needed for example when it is desired to plate certain copper surfaces (e.g., contact fingers) with nickel and/or gold to improve conductivity, or when it is desired to apply a solder mask over bare copper surfaces (SMOBC processes), or when it may be necessary simply to treat a reject piece in an effort to recover and re-use the underlying copper material. Also, while particularly apropos of printed circuit board manufacture, the need to strip away tin or tin-lead layers from copper surfaces also arises in other contexts where tin or tin-lead has been applied over a copper surface for decorative and/or functional purposes. Still further, needs may arise for stripping tin or tin-lead alloy layers from substrate surfaces other than copper, be they metallic or non-metallic surfaces.
Aqueous compositions designed to strip tin and/or tin-lead coatings from substrate surfaces, particularly copper surfaces, are known in the art. One class of such compositions includes those based upon hydrogen peroxide and hydrofluoric acid or a fluoride. See, e.g., U.S. Pat. Nos. 3,926,699; 3,990,982; 4,297,257; 4,306,933; 4,374,744 and 4,673,521. Another class involves those employing nitro-substituted aromatic compounds as a principal ingredient, often in conjunction with an inorganic acid (see, e.g., U.S. Pat. Nos. 3,677,949; 4,004,956; and 4,397,753) or an organic acid (see U.S. Pat. No. 4,439,338 disclosing the use of alkylsulfonic acids). Other known stripper compositions and processes are described in U.S. Pat. Nos. 4,424,097 and 4,687,545. Nitric acid-based strippers also have long been used in the art. See, e.g., the discussion in U.S. Pat. No. 4,713,144, and the use therein of a composition of nitric acid, sulfamic acid and ferric nitrate.
Irrespective of the particular type of stripping composition employed for removing tin or tin-lead alloy layers from a substrate, at some point the aqueous composition will undergo a decrease in its stripping effectiveness as stripped tin and/or lead species accumulate therein. The composition at that point can be discarded as waste, provided of course that suitable waste treatment methods are employed to insure that environmentally disadvantageous components are first removed and/or converted into environmentally acceptable form. More advantageous still would be to regenerate the aqueous composition so as to restore its stripping effectiveness. This is particularly attractive to those users of the aqueous stripping compositions who might not have adequate waste treatment systems on the premises since it would eliminate their need to arrange for hauling of potentially hazardous materials.
Regeneration as such can be quite complicated. For example, in ammonium bifluoride-hydrogen peroxide stripping compositions for tin or tin-lead, the aqueous composition can be regenerated (more accurately, replenished) by periodic additions of hydrogen peroxide to maintain its concentration above particular set levels required for effective stripping. Replenishment in this manner cannot, however, be effected indefinitely since eventually tin and/or lead and/or other complex metallic species build up to a degree which requires removal before stripping can continue effectively. Here again, the safe removal/disposal of these impurities is often not an easy matter, and, indeed, it would be far more economical if the metal values could somehow be easily recovered in saleable form.
A discussion of replenishment/regeneration of fluorine-containing solder stripping solutions (e.g., hydrogen peroxide-ammonium bifluoride-type baths) can be found in U.S. Pat. No. 4,673,521. The regeneration taught there involves addition of potassium ions to the solution to form a solid potassium-tin compound which can be separated from the solution. More particularly, the regeneration process involves filtering to remove sludge, addition of lime to separate lead, addition of potassium ions, filtering to remove the precipitated tin-potassium compound, passage through a chelating ion exchange resin (H.sup.+ form) to remove copper ions, and then replenishment with additions of ammonium bifluoride and hydrogen peroxide to desired concentrations. The process is, thus, quite time consuming and still results in complex metal compounds which require further treatment.